The present invention relates to ceramic articles and, more particularly, to laminate ceramic stand-alone articles or coatings having novel material architecture, properties and uses. Most particularly, the present invention relates to ceramic/metal laminates having superior thermal shock induced crack formation resistance.
When monolithic ceramics are subjected to high heat transfer or rapid changes in temperature, damage in the form of cracks occurs due to high tensile stresses, inherent low fracture toughness, and the presence of processing flaws [1-4].
Quantitatively, thermal shock behavior is characterized by several material parameters. One group is combined into the thermal stress resistance parameter, R, in centigrade, expressed in terms of the ultimate tensile stress, "sgr"cr, the biaxial elastic modulus, Exe2x80x2=E/(1xe2x88x92xcexd), and the thermal expansion coefficient, xcex1:
R="sgr"cr/Excex1xe2x80x83xe2x80x83(1)
R is the lower bound for the maximum difference in temperature that a ceramic can withstand without cracking. Another parameter, known as the Biot modulus, xcex2, reflects the severity of the thermal shock conditions:
xcex2=th/kxe2x80x83xe2x80x83(2)
where t is the length scale (most often the thickness), h the surface heat transfer, and k the thermal conductivity [5].
Quenching in water generates severe conditions for thermal shock due to a phase transformation from water to vapor, hence the heat transfer is high. On the other hand, water quench presents some difficulties, which arise from the non-uniform heat transfer, which is a strong function of the local temperature [6]. The critical temperature, xcex94TC, is usually defined [4] as the temperature difference between the maximum temperature of the specimen (or even the furnace temperature) and the temperature of the quenched media prior to cracking. This critical temperature as a function of specimen thickness for several ceramics quenched in water was determined experimentally [4], and demonstrated increased critical thermal difference as the specimen""s thickness reduced. The severity of quenching in room temperature water compared with cooling in boiling water or other hot liquids was shown [4, 7].
While some materials, such as ordinary glass, can take a temperature shock of only 80xc2x0 C. before cracks initiate, others, like Silicon Nitride [4] can withstand sudden changes of more than 600xc2x0 C. Therefore, monolithic ceramics are not sufficiently strong to serve at high temperatures and in harsh environments when thermal shock must be taken into account [8].
It is well known that layered structure shells, such as nacre, are much s tougher than polycrystalline calcite [9, 10]. Recent work has shown that layered ceramic structures can absorb much higher mechanical energies even when individual ceramic layers experienced massive cracking at room [11, 12] and high temperatures [13]. A cost-effective way of making toughened ceramic laminate for use at high temperatures under bending was presented [14, 15]. A similar constitution, but with stronger interfaces, has been suggested lately [16]. Work by Marshall et al. [17] on alumina interlayers separating ceria doped zirconia shows that the presence of the interlayers prevents the spread of the damage zone ahead of the crack tip, while at the same time causing the transformation zone to spread laterally. This effect permits a considerable increase in the work that is absorbed by that zone.
One of the most efficient ways of absorbing energy is by plastic deformation of the material. Metal-ceramic laminates have been studied extensively [18-21] in this context. Such materials exhibit many attractive properties, but the relatively poor oxidation resistance of the metals, compared with that of ceramics, makes such materials unsuitable; for use at high temperatures.
There is thus a widely recognized need for, and it would be highly advantageous to have, ceramic material having an architecture devoid of the above limitations and which can withstand higher thermal shocks and which can therefore better suited for implementation in thermal shock involving applications.
A new material architecture with a view to increase the maximum difference in temperature that ceramics can withstand and still maintain structural integrity and strength is described herein. The new architecture is of ceramic layers alternating with metallic interlayers. In this system, the ceramic is the high melting point constituent, possessing high stiffness, high wear and fatigue resistance, and able to maintain these properties in a corrosive environment and at elevated temperatures. The metal interlayers provide the needed compliance, ductility, and toughness. The interface should be strong enough to prevent crack deflection and disintegration of the material.
Thus, according to one aspect of the present invention there is provided a laminate article of manufacture comprising a plurality of ceramic layers interposed by at least one metallic interlayer, such that a ductile behavior with energy dissipating feature is obtained.
According to yet another aspect of the present invention there is provided a laminate article of manufacture comprising a plurality of ceramic layers interposed by at least one metallic interlayer, such that a driving force in the laminate article is at least twice as much as compared with a similar monolithic ceramic article.
According to still another aspect of the present invention there is provided a laminate article of manufacture comprising a plurality of ceramic layers interposed by at least one metallic interlayer, such that a crack arrest mechanism is formed at ceramic-metal interfaces, thereby reducing crack formation when exposed to thermal shock, as compared to a similar monolithic ceramic article.
According to an additional aspect of the present invention there is provided a laminate article of manufacture comprising a plurality of ceramic layers interposed by at least one metallic interlayer, such that substantially no interaction between cracking mechanisms developing in one ceramic layer with those in adjacent ceramic layers are evident upon thermal shock.
According to yet an additional aspect of the present invention there is provided a laminate article of manufacture comprising a plurality of ceramic layers interposed by at least one metallic interlayer, such that a cracking mechanism in each ceramic layer is substantially an independent event associated with a maximum tensile stresses in that layer.
According to still an additional aspect of the present invention there is provided a laminate article of manufacture comprising a plurality of ceramic layers interposed by at least one metallic interlayer, such that a residual load to fracture of the laminated article following a thermal shock is at least two fold higher as compared to a similar monolithic ceramic article.
According to a further aspect of the present invention there is provided an article of manufacture comprising a bulk ceramic body being coated with a coat including a plurality of ceramic layers interposed by at least one metallic interlayer, such that such that a ductile behavior with energy,dissipating feature is obtained for the coat.
According to yet a farther aspect of the present invention there is provided an article of manufacture comprising a bulk ceramic body being coated with a coat including a plurality of ceramic layers interposed by at least one metallic interlayer, such that a crack arrest mechanism is formed at ceramic-metal interfaces, thereby reducing crack formation in the article when exposed to thermal shock, as compared to a similar monolithic ceramic article.
According to still a further aspect of the present invention there is provided an article of manufacture comprising a bulk ceramic body being coated with a coat including a plurality of ceramic layers interposed by at least one metallic interlayer, such that substantially no interaction between cracking mechanisms developing in one ceramic layer with those in adjacent ceramic layers are evident upon thermal shock.
According to an additional aspect of the present invention there is provided an article of manufacture comprising a bulk ceramic body being coated with a coat including a plurality of ceramic layers interposed by at least one metallic interlayer, such that a cracking mechanism in each ceramic layer is substantially an independent event associated with a maximum tensile stresses in that layer.
According to yet an additional aspect of the present invention there is provided an article of manufacture comprising a bulk ceramic body being coated with a coat including a plurality of ceramic layers interposed by at least one metallic interlayer, such that a residual load to fracture of the article following a thermal shock is at least two fold higher as compared to a similar monolithic ceramic article.
According to further features in preferred embodiments of the invention described below, the ceramic body (where applicable) and each of the plurality of ceramic layers are each independently of a ceramic material selected from the group consisting of Al2O3, SiC, AlN, B4C, ZrO2, Glass-Ceramics and Si3N4.
According to still further features in the described preferred embodiments the at least one metallic interlayer is selected from the group consisting of a metal and an alloy.
According to still further features in the described preferred embodiments the at least one metallic interlayer is of a metallic material selected from the group consisting of Ni, Al, Ti, Cu, Hf, Nb, Mo, Cr, Ta, Re, Rh and alloys thereof.
According to still further features in the described preferred embodiments the at least one metallic interlayer is of a brazing alloy.
According to still further features in the described preferred embodiments the brazing alloy is selected from the group consisting of CUSIL ABA (63% Ag, 35.25% Cu, 1.75% Ti), INCUSIL (59% Ag, 27.25% Cu, 12.5% In, 1.25% Ti), TICUSIL (68.8% Ag, 26.7% Cu, 4.5% Ti) and COPPER ABA (92.75% Cu, 3% Si, 2% Al, 2.25% Ti).
According to still further features in the described preferred embodiments the plurality of ceramic layers and the at least one metallic interlayer are layered by a bonding method selected from the group consisting of liquid state bonding, solid state bonding and transient liquid phase bonding.
According to still further features in the described preferred embodiments at least two of the plurality of ceramic layers are of a single ceramic material.
According to still further features in the described preferred embodiments at least two of the ceramic layers are of different ceramic materials.
According to still further features in the described preferred embodiments the at least one metallic interlayer includes a plurality of metallic interlayers.
According to still further features in the described preferred embodiments at least two of the plurality of metallic layers are of a single metallic material.
According to still further features in the described preferred embodiments at least two of the plurality of metallic layers are of different metallic materials.
According to still further features in the described preferred embodiments the at least one metallic layer is thinner that each of the plurality of ceramic layers.
The present invention successfully addresses the shortcomings of the presently known configurations by providing an article of manufacture or an article including same which is useful and superior in thermal shock involving applications.